Microelectronic devices with improved heat dissipation and methods for cooling microelectronic devices

ABSTRACT

Microelectronic devices with improved heat dissipation, methods of making microelectronic devices, and methods of cooling microelectronic devices are disclosed herein. In one embodiment, the microelectronic device includes a microelectronic substrate having a first surface, a second surface facing opposite from the first surface, and a plurality of active devices at least proximate to the first surface. The second surface has a plurality of heat transfer surface features that increase the surface area of the second surface. In another embodiment, an enclosure having a heat sink and a single or multi-phase thermal conductor can be positioned adjacent to the second surface to transfer heat from the active devices.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/608,648 filed Dec. 8, 2006, now U.S. Pat. No. 8,291,966, which is adivisional of U.S. patent application Ser. No. 10/767,232 filed Jan. 28,2004, now U.S. Pat. No. 7,183,133, which is a divisional of U.S. patentapplication Ser. No. 10/228,906 filed Aug. 27, 2002, now U.S. Pat. No.6,710,442, each of which is incorporated herein by reference in itsentirety.

TECHNICAL FIELD

The present invention is directed toward microelectronic devices withimproved heat dissipation, methods of making microelectronic devices,and methods of cooling microelectronic devices.

BACKGROUND

The current trend in microelectronic device fabrication is tomanufacture smaller and faster microelectronic devices for computers,cell phones, pagers, personal digital assistants, and many otherproducts. All microelectronic devices generate heat, and rejection ofthis heat is necessary for optimum and reliable operation. As the speedand capacity of microelectronic devices has increased, the integratedcircuitry of the devices has become smaller and more closely spaced,thereby generating more heat. Moreover, the cooling space within themicroelectronic devices has become smaller. Accordingly, heatdissipation has become a critical design factor.

FIGS. 1A-1C schematically illustrate an existing method for dissipatingheat from devices formed on a wafer 110. FIG. 1A is a schematic sideview of the wafer 110, and FIG. 1B is a schematic side view of the wafer110 thinned, for example, in accordance with the procedures disclosed inU.S. Pat. No. 6,180,527, assigned to the assignee of the presentinvention and incorporated herein by reference. Thinning the wafer 110increases the surface area per unit volume of the wafer 110, andtherefore the ability of the wafer 110 to reject heat. FIG. 1Cillustrates a microelectronic device 100 including a portion of thediced wafer 110, such as a microelectronic die 140, and a heat sink 130attached to the die 140. In operation, the heat sink 130 absorbs heatfrom the die 140 and dissipates the heat into the ambient air. In oneembodiment, a cooling fan can be added to force air past the heat sink130.

Another method for dissipating heat from the die 140 includes attachinga heat pipe (not shown) to the surface of the die 140. A heat pipetypically includes a closed, evacuated vessel with a working fluidinside. One end of the heat pipe is positioned to absorb heat from thedie 140. The heat causes the fluid in the heat pipe to vaporize andcreate a pressure gradient in the pipe. This pressure gradient forcesthe vapor to flow along the heat pipe to a cooler section where itcondenses, giving up its latent heat of vaporization. The cooler sectionof the heat pipe then dissipates the heat into the ambient air. Theworking fluid then returns to the end of the heat pipe proximate to thedie 140.

The foregoing heat dissipation methods have several drawbacks. Forexample, attaching a heat sink, heat pipe, and/or cooling fan to themicroelectronic device may substantially increase the weight and/or sizeof the device. Furthermore, the limited contact area between the die andthe heat sink or heat pipe may limit the heat transfer between thedevices.

SUMMARY

The present invention is directed toward microelectronic devices withimproved heat dissipation, methods of making microelectronic devices,and methods of cooling microelectronic devices. In one aspect of theinvention, a microelectronic device includes a microelectronic substratehaving a first surface, a second surface facing opposite from the firstsurface, and a plurality of active devices at least proximate to thefirst surface. The second surface has a plurality of heat transfersurface features. In a further aspect of the invention, the secondsurface has a projected area and a surface area, including the heattransfer surface features, that is greater than the projected area. Inyet a further aspect of the invention, the heat transfer surfacefeatures are not configured to provide electrical communication betweenthe microelectronic substrate and components external to themicroelectronic substrate. In a further aspect of the invention, theheat transfer surface features are integrally formed in the secondsurface.

In another aspect of the invention, the second surface of themicroelectronic substrate defines at least in part a thermal conductorvolume. The microelectronic device further includes an enclosure membersealably coupled to the microelectronic substrate, and a thermalconductor disposed within the thermal conductor volume to transfer heatfrom the active devices. In a further aspect of the invention, thesecond surface has a plurality of recesses. The microelectronic devicefurther includes a sealed heat transport system coupled to the secondsurface of the microelectronic substrate. The heat transport system hasa cavity with a thermal conductor configured to transfer heat from themicroelectronic substrate to a region external to the microelectronicsubstrate, and the thermal conductor is sealably excluded from therecesses.

In another aspect of the invention, a method for making themicroelectronic device includes forming active devices at leastproximate to the first surface of the microelectronic substrate, andremoving material from the second surface of the microelectronicsubstrate to form heat transfer surface features. In a further aspect ofthe invention, the method includes forming at least one recess in thesecond surface of the microelectronic substrate, and disposing thethermal conductor in the at least one recess. The thermal conductor isnot configured to provide electrical communication between themicroelectronic substrate and external components. The method furtherincludes sealably enclosing the at least one recess with the thermalconductor positioned to transfer heat from the active devices to aregion external to the microelectronic substrate.

In yet another aspect of the invention, a method for cooling themicroelectronic device includes providing a microelectronic substratehaving a first surface, a second surface with at least one recess, and aplurality of active devices at least proximate to the first surface. Themethod further includes vaporizing at least some of a liquid positionedin the at least one recess as the liquid absorbs heat from themicroelectronic substrate, and condensing at least some of the vaporizedliquid by transferring heat away from the microelectronic substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic side view of a wafer in accordance with the priorart.

FIG. 1B is a schematic side view of the wafer shown in FIG. 1A afterthinning in accordance with the prior art.

FIG. 1C is a schematic side view of a microelectronic device including aportion of the wafer and a heat sink in accordance with the prior art.

FIG. 2A is a schematic side view of a substrate positioned forprocessing in accordance with an embodiment of the invention.

FIG. 2B is a schematic side view of a microelectronic device formed fromthe substrate shown in FIG. 2A having heat transfer surface features inaccordance with an embodiment of the invention.

FIG. 3 is a schematic side view of a microelectronic device having heattransfer surface features in accordance with another embodiment of theinvention.

FIG. 4 is a schematic side view of a microelectronic device having heattransfer surface features in accordance with yet another embodiment ofthe invention.

FIG. 5 is a schematic side view of a microelectronic device including aheat dissipation system in accordance with another embodiment of theinvention.

FIG. 6 is a schematic side view of a microelectronic device including asealed heat transport system in accordance with another embodiment ofthe invention.

DETAILED DESCRIPTION

The present disclosure describes microelectronic devices with increasedheat dissipation, methods for manufacturing microelectronic devices, andmethods for cooling microelectronic devices. Many specific details ofseveral embodiments of the invention are set forth in the followingdescription and in FIGS. 2A-6 to provide a thorough understanding ofsuch embodiments. Those of ordinary skill in the art, however, willunderstand that the invention can have additional embodiments, and thatthe invention may be practiced without several of the details describedbelow.

FIG. 2A is a schematic side view of a microelectronic substrate 240,such as a wafer, prior to processing in accordance with an embodiment ofthe invention. The microelectronic substrate 240 can include a firstsurface 213 and a second surface 216 facing opposite from the firstsurface 213. As used herein, the term “microelectronic substrate”includes substrates (such as wafers and/or dies diced from wafers) uponwhich and/or in which microelectronic circuits or components, datastorage elements or layers, and/or vias or conductive lines are or canbe fabricated. The microelectronic substrate 240 can include activedevices 220, such as capacitors, transistors, and/or memory cells,proximate to the first surface 213. In operation, the active devices 220generate heat that must be dissipated to obtain the desired operationalcharacteristics from the devices 220.

FIG. 2B is a schematic side view of a microelectronic device 200, thatincludes the microelectronic substrate 240 and a plurality of heattransfer surface features 250 in accordance with one embodiment of theinvention. In one aspect of this embodiment, the microelectronicsubstrate 240 can be modified (for example, by thinning themicroelectronic substrate 240 in accordance with procedures described inU.S. Pat. No. 6,180,527) before the formation of the heat transfersurface features 250. In other embodiments, the microelectronicsubstrate 240 is not thinned. In either embodiment, the heat transfersurface features 250 increase the surface area of the second surface 216of the microelectronic substrate 240. For example, the second surface216 has a projected area PA generally parallel to the first surface 213.The surface area of the second surface 216 including the heat transfersurface features 250 is greater than the projected area PA and thisincreased surface area can enhance the ability of the microelectronicsubstrate 240 to dissipate heat.

In one embodiment, the heat transfer surface features 250 includegrooves 261 that permit a heat transfer medium, such as air, to haveincreased thermal contact with the microelectronic substrate 240,thereby increasing the amount of heat transferred from themicroelectronic substrate 240. In one aspect of this embodiment, eachheat transfer surface feature 250 includes a first heat transfer wall252, a second heat transfer wall 254, and a portion of the secondsurface 216. In a further aspect of this embodiment, the first andsecond heat transfer walls 252 and 254 can be generally parallel, andeach wall can extend from the second surface 216 to a recessed surface256. In other embodiments, (such as an embodiment described below withreference to FIG. 4), the heat transfer walls can be non-parallel.

In a further aspect of an embodiment illustrated in FIG. 2B, each heattransfer surface feature 250 has a generally similar shape and size. Forexample, the heat transfer surface features 250 can be disposed in aportion of the microelectronic substrate 240 having a thickness T, andeach surface feature 250 can have a depth D and a width W₁, and can bespaced apart from the adjacent surface feature 250 by a distance W₂. Inone embodiment, D is approximately 400 microns and T is approximately750 microns. In another embodiment, D is from about one-fifth to aboutthree-fourths the value of T. In still a further embodiment, D is fromabout one-third to about one-half the value of T. In additionalembodiments, D, T, W₁, and W₂ can have other values depending on theheat transfer requirements and available volume of the microelectronicsubstrate 240.

In one embodiment, the heat transfer surface features 250 are notconfigured to provide electrical communication between themicroelectronic substrate 240 and components external to themicroelectronic substrate 240. This function can instead be provided byelectrical couplers 208, such as solder balls, which are electricallycoupled to the active devices 220. In another embodiment, the heattransfer surface features 250 can be integrally formed in themicroelectronic substrate 240. For example, the heat transfer surfacefeatures 250 can be formed by removing material from the second surface216 (such as by etching) to create the grooves 261. In otherembodiments, the heat transfer surface features 250 can be formed bydepositing material on the second surface 216.

In operation, the active devices 220 generate heat that flows throughthe microelectronic substrate 240, primarily by conduction, and istransferred away from the substrate 240 by convection, conduction and/orradiation. For example, a heat transfer medium, such as air, can moveproximate to the heat transfer surface features 250 to absorb heat fromthe microelectronic substrate 240. The heat transfer surface features250 increase the rate at which heat is transferred away from themicroelectronic substrate 240 by increasing the surface area of thesubstrate 240. One advantage of this arrangement is that themicroelectronic substrate 240 can be cooled without the addition of aheat sink. In other embodiments (such as those described below withreference to FIGS. 5 and 6), heat sinks can further increase the rate atwhich heat is transferred from the microelectronic substrate.

FIG. 3 is a schematic side view of a microelectronic device 300 havingheat transfer surface features 350 in accordance with another embodimentof the invention. In one aspect of this embodiment, the microelectronicdevice 300 includes a microelectronic substrate 340 having a firstsurface 313, a second surface 316, and a plurality of heat transfersurface features 350 formed in the second surface 316 to increase thesurface area and enhance the ability of the substrate 340 to dissipateheat. Grooves 361 separate the heat transfer surface features 350 fromeach other. Each heat transfer surface feature 350 can include a firstheat transfer wall 352, a second heat transfer wall 354, and a portionof the second surface 316. In the illustrated embodiment, the first andsecond heat transfer walls 352 and 354 are generally parallel, and eachwall 352 and 354 extends from the second surface 316 to a recessedsurface 356. The recessed surface 356 has an arcuate shape, curvingbetween the first heat transfer wall 352 of one heat transfer surfacefeature 350 and the second heat transfer wall 354 of an adjacent heattransfer surface feature 350.

FIG. 4 is a schematic side view of a microelectronic device 400 havingheat transfer surface features 450 in accordance with another embodimentof the invention. In one aspect of this embodiment, the microelectronicdevice 400 includes a microelectronic substrate 440 having a firstsurface 413, a second surface 416, and a plurality of heat transfersurface features 450 formed in the second surface 416. Grooves 461separate the heat transfer surface features 450 from each other. Eachheat transfer surface feature 450 can include a first heat transfer wall452, a second heat transfer wall 454, and a portion of the secondsurface 416. In the illustrated embodiment, the first heat transfer wall452 of one heat transfer surface feature 450 and the second heattransfer wall 454 of an adjacent heat transfer surface feature 450 arenonparallel to each other. The distance between the first heat transferwall 452 and the second heat transfer wall 454 decreases as the walls452 and 454 extend from the second surface 416 toward the first surface413. In other embodiments, heat transfer surface features with othershapes and/or sizes can be used to increase the surface area of themicroelectronic substrate.

FIG. 5 is a schematic side view of a microelectronic device 500including a microelectronic substrate 540 having active devices 220 anda heat dissipation system 502. In the illustrated embodiment, themicroelectronic substrate 540 is similar to the microelectronicsubstrate 240 discussed above with reference to FIG. 2B. For example,the microelectronic substrate 540 can include heat transfer surfacefeatures 550, separated by grooves 561, and having first heat transferwalls 552, second heat transfer walls 554, and recessed surfaces 556recessed from a second surface 516. In other embodiments, themicroelectronic substrate 540 can have other configurations, such asthose discussed above with reference to FIGS. 3 and 4. In any of theseembodiments, the heat dissipation system 502 can transfer heat from themicroelectronic substrate 540 to an external heat sink 589. For example,the heat dissipation system 502 of the illustrated embodiment caninclude a thermal conductor 572 disposed within a thermal conductorvolume 570 and arranged to transfer heat to the external heat sink 589.The thermal conductor volume 570 can be defined by a first side wall580, a second side wall 581, an enclosure member 584, and portions ofthe microelectronic substrate 540, such as the first and second heattransfer walls 552 and 554 and the recessed surfaces 556 of the heattransfer features 550.

The thermal conductor 572 is positioned within the thermal conductorvolume 570 to absorb heat from the microelectronic substrate 540. In theillustrated embodiment, the thermal conductor 572 has multiple phaseswithin the thermal conductor volume 570. For example, the thermalconductor 572 can include a liquid phase portion 578 disposed primarilywithin the grooves 561, and a gas phase portion 574 disposed primarilybetween the first side wall 580 and the second side wall 581. In oneembodiment, the thermal conductor 572 can include water, ammonia, and/oralcohol. In other embodiments, the thermal conductor 572 can includeother substances and/or can have other single or multi-phasecompositions. For example, the thermal conductor 572 can include only asolid phase material, or only a gas-phase material, or a portion of thethermal conductor 572 can include a solid phase and another portion caninclude a liquid or a gas phase. In one embodiment, the thermalconductor volume 570 can have a negative gauge pressure so that thethermal conductor 572 can more easily vaporize. In other embodiments,the thermal conductor volume 570 can have other pressures. In any ofthose embodiments, the thermal conductor 572 can transfer heat from theheat transfer surface features 550 to the enclosure member 584.

The enclosure member 584 absorbs heat from the thermal conductor 572 andtransfers the heat to the external heat sink 589. In the illustratedembodiment, the enclosure member 584 is sealably coupled to the firstand second side walls 580 and 581, which are integral portions of themicroelectronic substrate 540. In other embodiments, the first andsecond side walls 580 and 581 can be integral portions of the enclosuremember 584, or the walls 580 and 581 can initially be separate from theenclosure member 584 and the microelectronic substrate 540. In any ofthese embodiments, the external heat sink 589 can include a plurality offins 586 to transfer heat to an external medium, such as the surroundingair. In the illustrated embodiment, the external medium can move throughchannels 588 between the heat fins 586. In other embodiments, the heatdissipation system 502 may not include heat fins 586.

In operation, the active devices 220 generate heat that flows throughthe microelectronic substrate 540 to the heat transfer surface features550. Accordingly, the first and second heat transfer walls 552 and 554,the recessed surfaces 556, and the second surface 516 transfer heat tothe thermal conductor 572. In the illustrated embodiment, the liquidphase portion 578 of the thermal conductor 572 in the grooves 561absorbs heat from the first and second heat transfer walls 552 and 554and the recessed surfaces 556. As the liquid phase portion 578 absorbsheat, it vaporizes and transforms into the gas phase portion 574. Thegas phase portion 574 may form proximate to the recessed surface 556because the surface 556 is close to the hot active devices 220. Afterformation, the gas phase portion 574 rises out of the liquid phaseportion 578 toward the enclosure member 584. As the gas phase portion574 approaches and/or contacts the enclosure member 584, the gas phaseportion 574 transfers heat to the enclosure member 584 and condenses.Drops of condensate 576 fall downward toward the grooves 561 and formpart of the liquid phase portion 578 in the grooves 561. As the processrepeats, heat is transferred from the microelectronic substrate 540 tothe enclosure member 584 and from the enclosure member 584 to theenvironment external to the microelectronic substrate 540. An advantageof this arrangement is the rate at which heat is transferred from themicroelectronic substrate 540 can be increased due to the large surfacearea added by the heat sink 589 and the heat transfer surface features550.

The heat dissipation system 502 of the illustrated embodiment caninclude wicks 582 to ensure that the liquid phase portion 578 of thethermal conductor 572 returns proximate to the microelectronic substrate540 independent of the orientation of the system 502 with respect togravitational forces, or other forces, such as centrifugal forces. Inone aspect of this embodiment, the wicks 582 are positioned proximate tothe first and second side walls 580 and 581. If the microelectronicdevice 500 is oriented such that gravity does not pull the drops ofcondensate 576 toward the grooves 561, the wicks 582 will return theliquid phase portion 578 to the microelectronic substrate 540 viacapillary action. Other embodiments, such as those in which the grooves561 are consistently oriented beneath the enclosure member 584, may notinclude wicks 582.

FIG. 6 is a schematic side view of a microelectronic device 600 in whichthe microelectronic substrate 540 attached to a sealed heat transportsystem 602. The sealed heat transport system 602 transfers heat from themicroelectronic substrate 540 to the external heat sink 589. Suitablesealed heat transport systems 602 include Therma-Base Heat Sinksmanufactured by Thermacore International, Inc. of Lancaster, Pa. In oneembodiment, the sealed heat transport system 602 absorbs heat from themicroelectronic substrate 540 via a first thermal conductor 572 anddissipates the heat via the external heat sink 589. In one embodiment,the external heat sink 589 can include fins 586 and in otherembodiments, the fins 586 are eliminated. In any of these embodiments,the sealed heat transport system 602 can include an internal, single ormulti-phase second thermal conductor 672 that transfers heat from thefirst thermal conductor 572 and the microelectronic substrate 540 to theexternal heat sink 589. For example, the second thermal conductor caninclude a liquid phase portion 678, a gas phase portion 674 and aprecipitate portion 676.

The sealed heat transport system 602 can be attached to themicroelectronic substrate 540 proximate to the grooves 561. In oneembodiment, the sealed heat transport system 602 is attached to thesecond surface 516 of the microelectronic substrate 540 with anadhesive, such as a nitride adhesive that prevents the first thermalconductor 572 from leaking out of the grooves 561. In one aspect of thisembodiment, the first thermal conductor 572 can include the liquid phaseportion 578 and the gas phase portion 574, as described above withreference to FIG. 5. In other embodiments, the first thermal conductor572 can have other single or multi-phase compositions, also as describedabove with reference to FIG. 5. In any of these embodiments, the firstthermal conductor 572 is physically isolated with a membrane from thesecond thermal conductor 672, but can transfer heat to the secondthermal conductor 672.

In operation, the active devices 220 generate heat which flows throughthe microelectronic substrate 540 to the heat transfer surface features550. A first surface 614 of the sealed heat transport system 602 absorbssome of the heat directly from the microelectronic substrate 540. Thefirst thermal conductor 572 in the grooves 561 also absorbs heat throughthe first heat transfer walls 552, the second heat transfer walls 554,and the recessed surfaces 556. In the illustrated embodiment, the liquidphase portion 578 absorbs heat and vaporizes, forming the gas phaseportion 574. The heated first thermal conductor 572 transfers the heatto the first surface 614 of the sealed heat transport system 602. As theprocess repeats, heat is transferred from the microelectronic substrate540 to the sealed heat transport system 602, and from the sealed heattransport system 602 to the external heat sink 589.

From the foregoing, it will be appreciated that specific embodiments ofthe invention have been described herein for purposes of illustration,but that various modifications may be made without deviating from thespirit and scope of the invention. Accordingly, the invention is notlimited except as by the appended claims.

We claim:
 1. A method of making a microelectronic device, comprising: removing material from a back-side surface of a microelectronic substrate to form recesses separate from active devices of the microelectronic substrate, wherein the back-side surface has a projected area, and wherein a surface area of the back-side surface including a surface area of the recesses is greater than the projected area; forming thermal conductors at the individual recesses, wherein the thermal conductors do not provide electrical communication with the active devices or external components; and sealably enclosing the thermal conductors by forming a barrier coupled between a first portion of the back-side surface and a second portion of the back-side surface.
 2. The method of claim 1 wherein removing the material from the back-side surface includes etching grooves in the back-side surface.
 3. The method of claim 1 wherein the barrier encloses the thermal conductors such that all of the thermal conductors are enclosed within a common cavity.
 4. The method of claim 1 wherein the barrier encloses the thermal conductors such that the individual thermal conductors are physically isolated from other thermal conductors.
 5. The method of claim 1 wherein forming the thermal conductors further includes disposing a liquid phase material in the individual recesses.
 6. The method of claim 5, further comprising disposing a gas phase material in the individual recesses.
 7. The method of claim 1 wherein forming the thermal conductors includes disposing a solid phase material in the individual recesses.
 8. A method of making a microelectronic device, comprising: forming at least one recess in a first surface of a microelectronic substrate facing away from a second surface of the microelectronic substrate, wherein the microelectronic substrate includes active devices at least proximate to the second surface of the microelectronic substrate; and sealably enclosing a thermal conductor in the recess, wherein the thermal conductor is not configured to provide electrical communication with the active devices or external components.
 9. The method of claim 8 wherein sealably enclosing the thermal conductor includes sealing an opening of the recess.
 10. The method of claim 8, further comprising disposing the thermally conductive material in the recess.
 11. The method of claim 10 wherein disposing the thermally conductive material includes disposing a solid phase material in the recess.
 12. The method of claim 10 wherein disposing the thermally conductive material includes disposing a fluid in the recess.
 13. A method of making a microelectronic device, comprising: forming heat transfer surface features in a first surface of a microelectronic substrate having active devices at least proximate to a second surface of the microelectronic substrate that is facing away from the first surface of the microelectronic substrate; and sealably enclosing the surface features in a casing having a cavity, wherein the cavity has a volume defined by an interior surface area of the casing and the first surface of the microelectronic substrate.
 14. The method of claim 13, further comprising disposing a fluid in the cavity.
 15. The method of claim 13, further comprising removing a gas phase fluid from the cavity to provide a negative gauge pressure in the cavity.
 16. The method of claim 13, further comprising pressurizing the cavity.
 17. The method of claim 13, further comprising forming a wicking surface in the cavity proximate an individual surface feature.
 18. The method of claim 13 wherein the surface features comprise recesses in the first surface, and wherein the method further comprises sealably enclosing individual recesses.
 19. The method of claim 18, further comprising disposing a fluid in the individual recesses.
 20. The method of claim 19 wherein the fluid comprises a first fluid, and wherein the method further comprises disposing a second fluid inside the cavity but outside the individual recesses. 